Prior fiber optic data communication techniques of multiplexing data onto a laser beam propagated in a fiber optic waveguide use two basic methods, direct modulation and external modulation, both of which have been complex and expensive. Direct modulation methods break the integrity of the fiber optic waveguide in the transmission link to introduce additional modulation components. Prior methods using direct modulation require an optical receiver, optical laser transmitter, and complex timing and formatting circuits to accomplish the introduction of additional data onto the laser carrier propagated through the fiber optic waveguide. The optical receiver serves to terminate the fiber optic waveguide and intercept the data stream modulated on the laser carrier. The formatting circuits add off-line external data and the timing regeneration circuits reconstruct the data stream with the additional data multiplexed therein. The reconstructed data is modulated on a new laser carrier generated and retransmitted by the transmitter. Such complex point-to-point segments are expensive and require significant amounts of power for operation. In distributed modulation systems, these factors have seriously limited the number of modulation sites that can be practically incorporated in a communication system where the modulation sites are spatially distributed along a fiber optic waveguide used as a laser transmission line. The power requirements of such systems are particularly important since they place severe operating life limits on remotely deployed portable applications which use batteries. With the complex circuits and significant battery requirements, the weight of such systems is another limiting factor, particularly when the system includes many modulators distributed along the fiber optic waveguide and must be carried by aircraft for deployment. Additionally, the complexity of the circuits reduces the inherent reliability of any such system.
Alternatively, applications of prior fiber optic communications techniques using external modulation have enjoyed only limited success since the techniques have employed inorganic crystal structures to achieve the modulation desired. Potassium dihydrogen phosphate (PDK) and lithium niobate (LiNbO.sub.3) crystals are two noncentrosymmetric crystalline structures found to achieve useable performance. The physical characteristics of such modulators limit their application, however. While the lithium niobate crystals offer some performance improvement over the PDK crystals, they are expensive and difficult to manufacture. Additionally, such crystals have characteristics which are less than ideally suited to the application. Their dielectric constants are high, their indices of refraction are not closely compatible with the optical fibers with which they interface, and intimate contact with the fibers is difficult to achieve thereby causing inefficient coupling into the optical fiber waveguide. The crystal structures also represent an optically lossy transmission medium. Such factors contribute to overall inefficient operation, physical limitations, and attendant high costs.
Doped polymers have been investigated as an alternative to the lithium niobate crystals in prior modulators but have failed to achieve performance that would permit their commercial application in optical communication systems. Prior doped polymers have been limited by sensitivity to temperature and subject to poor retention of molecular electrical dipole alignment (poling), resulting in gradual degradation of their noncentrosymmetry over time, thereby losing their electrooptical operating characteristics. Additionally, the dopants have not been absorbed into the polymers in sufficient density and have exhibited less than practical parametric variation from the applied electrical modulating fields.
Accordingly, there is a continuing need for an improved electrooptical modulating system permitting multiple modulator sites in a communication system which overcomes the above described shortcomings of prior art direct and external modulation methods. The present invention satisfies such a need.